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CHE1031
CHE1031 Lecture 2 HW Key
0.71 pt ea
Problems must be solved, or written out, in their entirety with all work shown on engineering
graph paper. You must label each set in the upper left hand corner with your name, the date
and the chapter. Problems must be identified by number and all work must be shown with
answers boxed. Be sure your handwriting is legible. An example is posted in the ‘course
basics’ section of our Moodle page.
Dalton’s atomic theory
1. Hydrogen sulfide is composed of two elements: hydrogen & sulfur. In an experiment,
6.500 g of hydrogen sulfide is fully decomposed into its elements.
a. If 0.384 g of hydrogen is isolated, how many grams of sulfur are isolated?
b. What fundamental law does this experiment demonstrate?
c. How does this law relate to Dalton’s atomic theory?
a. H2S has a percent composition of H (= (2.02/34.08)/100) = 5.9% and S (=
(32.06/34.08)/100) = 94.1%. So:
0.384 g = x g -> x = 6.12 g
5.9%
94.1%
b. The laws of conservation of mass, constant composition, and multiple proportions.
c. Dalton said that molecules formed when atoms combined at a constant ratio.
Molecules & formulas
2. What information does the empirical formula give us about a compound? What
information does the molecular formula add? What does the structural formula tell us?
Empirical: smallest (or simplest) ratio of elements in the molecule
Molecular: the number and type of atoms in the molecule (atomic shopping list)
Structural formula: atomic connectivity; which atom is connected to which atom
3. Determine the molecular and empirical formulas of the following:
a. the organic solvent, benzene, which has 6 carbons and 6 hydrogens
b. the compound silicon tetrachloride, with one silicon atom and four chlorine atoms,
used in making computer chips
c. the reactive substance, diborane, with two boron atoms and six hydrogen atoms
d. the sugar, glucose, with six carbon atoms, twelve hydrogen atoms and six oxygen
atoms
a. C6H6 molecular / CH empirical
b. SiCl4
c. B2H6 / BH3
d. C6H12O6 / CH2O
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4. How many hydrogen atoms are there in each of these molecules?
a. C2H5OH
5 + 1 = 6 H atoms
b. Ca(CH3COO)2
3 x 2 = 6 H atoms
c. (NH3)3PO4
3 x 3 = 9 H atoms
Discovery of Atomic Structure
5. How did Rutherford interpret the following observations made during his group’s particle
scattering experiments?
a. Most α-particles were not deflected as they passed through the gold foil?
b. A few α-particles were deflected.
c. What differences would you expect if gold foil were replaced with beryllium foil?
a. The vast majority of the atom’s volume is occupied only by the light electrons;
electrons are too light to deflect heavy α-particles. This volume comes to be known
as the electron cloud.
b. The few α-particles that hit the heavy protons concentrated in the very small nucleus
were deflected away from the atom’s nucleus.
c. If the atom in the foil were beryllium rather than gold the number of protons in the
nucleus would be four rather than 79. The nucleus would therefore be much less
dense and less likely to be hit by, or be dense enough to, deflect heavy α-particles.
6. An atom of rhodium (Rh) has a diameter of 2.7 x 10-8 cm.
a. How many Rh atoms would you place next to one another to span a distance of 6 μm?
b. If Rh atoms are spheres, what is the volume of a single atom (m3)?
a. 6 μm
1m
106 μm
102 cm 1 atom Rh
1m
= 2.22 x 104 = 20,000 atoms
2.7 x 10-8 cm
b. Volume of a sphere = 4/3πr3
radius = 2.7E-8 cm
1m
= 1.4E-10 m radius
2
1E2 cm
volume = (4/3)(π)((1.4E-10)3 m3) = 1 E-29 m3
7. Label each of these statements as true or false?
a. The nucleus of an atom contains most of the atom’s mass and takes up most of the
atom’s volume.
b. Every atom of an element has the same number of protons.
c. The number of electrons in an atom is equal to the number of neutrons in that atom.
d. The protons of a helium atom are held together by a force called the strong nuclear
force.
a. False: the nucleus is a tiny fraction of total volume.
b. True
c. False: number of electrons is equal to the number of protons.
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d. True
Sub-atomic particles & the periodic table
8. How many protons, neutrons & electrons are there in the following atoms:
a. 40Ar
b.
65
c.
70
18 p, 18 e-, n = 40 – 18 = 22
Zn
30 p, 30 e-, n = 65 – 30 = 35
Ga
31 p, 31 e-, n = 70 – 31 = 39
9. Each of these isotopes is used in medicine. How many protons & neutrons are there in
each isoptope?
a. phosphorous-32
15 p
n = 32 – 15 = 17
b. chromium-51
24 p
n = 51 - 24 = 27
c. cobalt-60
27 p
n = 60 – 27 = 33
Atomic Weight
1o. Rubidium has two naturally occurring isotopes, rubidium-85 (mass 84.9118 amu,
abundance 72.15%) and rubidium-87 (mass 86.9092 amu, abundance 27.85%). Calculate is
the average atomic mass of rubidium.
Average atomic mass = ((84.9118)(.7215) + (86.9092)(.2785)) = 61.26 + 24.20 = 85.46 amu
11. Mass spectroscopy is a great technique for identifying molecular and atomic substances
and samples. The spectrum of H2 gas can be measured under conditions that do not
cause the molecule to break down into H atoms. The two naturally occurring isotopes of
hydrogen are 1H (1.00783 amu; 99.9885%) and 2H (2.01410 amu; 0.0115%).
a. How many peaks with the mass spectrum of H2 have?
b. What is the approximate molecular masses of the types of H2 at each of these peaks?
c. Which type of H2 is most abundant (highest spectral peak) and which is least abundant
(lowest spectral peak)?
a. Three peaks of H2: 1-1, 1-2, 2-2
b. MW of the three H2 types: 2, 3, 4
c. The 1-1 is most abundant and gives the highest peak; 2-2 is least abundant & gives the
lowest spectral peak.
The Periodic Table
12. For each of these elements record the name, atomic number and identify it as a metal,
metalloid or nonmetal:
a. Li
lithium, 3, metal
b. Sc
scandium, 21, metal
c. Ge
germanium, 32, metalloid
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Ions & Charges
13. Use the periodic table to predict the charges of the following elements when they
become ions:
a. Sr
+2
b. As
-3
c. Br
-1
d. Se
-2
Ionic Compounds
14. Predict the formulas of the ionic compounds formed by these pairs of ions:
a. Cr+3 and Br-1
CrBr3
b. Fe+3 and O-2
c. Hg2
+2
and CO3
Fe2O3
-2
Hg(CO3)
15. Are these compounds ionic or molecular?
a. PF5
molecular
b. NaI
ionic
c. SCl2
molecular
d. Ca(NO3)2
ionic
Naming ionic compounds
Ionic compounds
16. Selenium, an element that is required in trace amounts for good nutrition, forms
compounds in a manner similar to sulfur. Name these ions:
a. SeO4-2
selenate
b. Se-2
selenide
c. HSehydrogen selenide
17. Give the charges of the cation and anion of each of these compounds:
a. CuS
+2/-2
b. Ag2(SO4)
+1/-2
c. Al(ClO3)3
+3/-1
18. Name these ionic compounds:
a. K(CN)
potassium cyanide
b. CoS
cobalt (II) sulfide
c. Sr(OH)2
strontium hydroxide
19. Write the formulas for these ionic compounds:
a. sodium phosphate
Na3(PO4)
b. zinc nitrate
Zn(NO3)
c. mercury (II) bromide
HgBr2
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Molecular compounds
20. Name each of these molecular compounds:
a. N2O
dinitrogen monoxide
b. NO
nitrogen monoxide
c. N2O5
dinitrogen pentoxide
Acids
21. Give the name or formula of each acid as appropriate:
a. hydroiodic acid
HI
b. H2CO3
carbonic acid
c. chloric acid
H(ClO3)
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